The major purpose of this project is to study the role of cell surface phenomena in the control of growth and differentiation. We have begun by developing a new method for observing dynamics of processes which occur on cell surfaces. Using this method we can determine rates of diffusion of particles from observations of the spontaneous fluctuations of their number in a small open region. At the present stage of our work we are concerned with perfecting our apparatus and methods, studies of diffusion in model membranes, and beginning studies on live cells. The apparatus consists of an optical system based on a conventional fluorescent microscope with data processing carried out by a correlation function computer of new design optimized for our experimental purposes. We are using our methods to study rates of diffusion of fluorescence labeled lipid analogues in planar lipid bilayer model membranes. We intend to use these studies to characterize the phase behavior of these membranes as a function of temperature and composition. We have begun our studies of transport on cell surfaces by measuring the mobility of the fluorescent labeled lectin, concanavalin A, on the surfaces of cultured myoblasts and fibroblasts. BIBLIOGRAPHIC REFERENCE: E. L. Elson and W. W. Webb, Ann. Rev. Biophys. and Bioeng., 4, 311 (1975). Concentration Correlation Spectroscopy: A New Biophysical Probe Based on Occupation Number Fluctuations.